On Wed, 21 Dec 2005 21:47:29 -0500, Peter Hansen <[EMAIL PROTECTED]> wrote:
>Bengt Richter wrote:
>[roughly "an inch is not exactly 25.4mm"]
>> At least according to my dusty 37th Edition Handbook of Chemistry and
>> Physics (c) 1955.
>> Maybe things have changed since then ;-)
>
>Wikipedia concurs with Jim, though it says the official change dates
>from 1958.
>
>Better throw that old book out, as it's also likely to be missing any
>reference to useful elements such as Lawrencium (1961), and Hassium
>(1984), not to mention Ununnilium, Ununumium and Ununbium (94, 94, 96
>respectively) or the most recently discovered element, which the PSU
>tried to supp
>
I had been using 25.4mm/inch myself, but decided to look it up, and
found that I had been using the "wrong" value -- now actually proving
to be right after all, after the definition change of 1958(1959?).
Google found an NIST page:
http://physics.nist.gov/Pubs/SP811/appenB.html
Where it says:
"""
B.6 U.S. survey foot and mile
The U. S. Metric Law of 1866 gave the relationship 1 m = 39.37 in (in is
the unit symbol for the inch). From 1893 until 1959, the yard was defined
as being exactly equal to (3600/3937) m, and thus the foot was defined as
being exactly equal to (1200/3937) m.
In 1959 the definition of the yard was changed to bring the U.S. yard and
the yard used in other countries into agreement. Since then the yard has
been defined as exactly equal to 0.9144 m, and thus the foot has been
defined as exactly equal to 0.3048 m. At the same time it was decided that
any data expressed in feet derived from geodetic surveys within the United
States would continue to bear the relationship as defined in 1893, namely,
1 ft = (1200/ 3937) m (ft is the unit symbol for the foot). The name of
this foot is "U.S. survey foot," while the name of the new foot defined in
1959 is "international foot." The two are related to each other through
the expression 1 international foot = 0.999 998 U.S. survey foot exactly.
In Sec. B.8 and Sec. B.9, the factors given are based on the international
foot unless otherwise indicated. Users of this /Guide/ may also find
the following summary of exact relationships helpful, where for
convenience the symbols /ft/ and /mi,/ that is, ft and mi in
italic type, indicate that it is the /U.S. survey foot/ or /U.S.
survey mile/ that is meant rather than the international foot (ft) or
international mile (mi), and where rd is the unit symbol for the rod and
fur is the unit symbol for the furlong.
1 /ft/ = (1200/3937) m
1 ft = 0.3048 m
1 ft = 0.999 998 /ft/
1 rd, pole, or perch = 16 1/2 /ft/
40 rd = 1 fur = 660 /ft/
8 fur = 1 U.S. survey mile (also called "statute mile") = 1 /mi/ = 5280 /ft/
1 fathom = 6 /ft/
1 international mile = 1 mi = 5280 ft
272 1/4 /ft/**2 = 1 rd**2
160 rd**2 = 1 acre = 43 560ft**2
640 acre = 1 /mi/**2
"""
(I changed italics to be indicated by /italic/ slashes, and superscript by **,
as well as changing special characters for a quarter and half to 1/4 and 1/2.
Hope I didn't typo ;-)
Anyway, 25.4 mm/inch it is. Nice to revert to that, after an unsettling
diversion ;-)
NIST ought to have it right, right? Or is there an intelligent design version
now? ;-/
Regards,
Bengt Richter
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